r. king   june 1977    subroutine ferctl
program control for computation of interferometry
observables, including both conventional vlbi delay and
delay rate (incoherent noise source) and differential
cycle count (coherent source).  ferctl calls ferstr, fermn,
or fersb depending on whether observed object is a star,
s/c in cislunar space, or s/c elsewhere in the solar system.
this is a revised version of old pep subroutine interf
written by m.ash/r.preston/r.king  feb 70 - jun 75

idopob= 0 when ferctl first called for observation
idopob= 1 in ferctl for differential n-count
observable on first call of ferctl
idopob= -1 set in ferctl on second call (ferctl called
second time only if idopob was set = 1 on
first call

nvel =0 positions determined
nvel =1 positions and velocities determined
norm =0 normals to sites not determined
norm =1 normals to sites determined
norm =2 normals to site determined, and partials
determined for solid body tides even if
lsite(1-3)=0

if save(48).ne.1  (default value), offset sample time
at second site by save(48) seconds.  for n-count obser-
vables, this compensates for the difference in propagation
time to the two sites. this difference is at most 20 ms, so
a common utrec is used to calculate at-utc, ct-at, and
precession-nutation at both sites.

ipct= 0 apply corrections to accumulated cycle-count
observable (corrections to delays calculated above
with ipct= -2,-1)
ipct= 1 calculate corrections to differential delay
ipct= 2 calculate corrections to differential delay rate

if jct(59)=0  eqnx(1-3) are corrections to observable
(delay in seconds, phase in degrees or cycles)
if jct(59)=1  eqnx(1-3) are clock terms (seconds), also
applied to utrec above